An air conditioner in which at least one outdoor unit and at least one indoor unit are mutually coupled by plural refrigerant pipes has been suggested. In the case where a temperature of an outdoor heat exchanger becomes equal to or less than 0° C. when this air conditioner performs a heating operation, the outdoor heat exchanger may be frosted. When the outdoor heat exchanger is frosted, ventilation to the outdoor heat exchanger is inhibited by the frost, and thus heat exchange efficiency in the outdoor heat exchanger may be degraded. Thus, when frosting occurs to the outdoor heat exchanger, a defrosting operation has to be performed to defrost the outdoor heat exchanger.
For example, in an air conditioner described in Patent Literature 1, an outdoor unit that includes a compressor, a four-way valve, an outdoor heat exchanger, an outdoor fan, and an outdoor expansion valve is coupled to two indoor units, each of which includes an indoor heat exchanger, an indoor expansion valve, and an indoor fan, via a gas refrigerant pipe and a liquid refrigerant pipe. In the case where, in this air conditioner, a defrosting operation is performed during a heating operation, the rotation of the outdoor fan and the rotation of the indoor fan are stopped. In conjunction with this, the compressor is stopped once, the four-way valve is switched such that the outdoor heat exchanger is shifted from a state of functioning as an evaporator (heating cycle) to a state of functioning as a condenser (cooling cycle), and the compressor is activated again. When the outdoor heat exchanger functions as the condenser, a high-temperature refrigerant discharged from the compressor flows into the outdoor heat exchanger and melts frost formed on the outdoor heat exchanger. Thus, the outdoor heat exchanger can be defrosted.
During the defrosting operation, however, as described above, a refrigerant circuit is switched from a heating cycle to the cooling cycle. For this reason, the heating operation is interrupted during the defrosting operation. Thus, a sense of comfort of the user may be hindered by frequent switching to the defrosting operation during the heating operation. In order to solve this problem, a typical air conditioner, in which a rotational speed of the outdoor fan is controlled so as to prevent the temperature (an evaporation temperature) of the outdoor heat exchanger during the heating operation from becoming equal to or less than a temperature (0° C.) at which the outdoor heat exchanger is frosted, is available. More specifically, evaporation pressure that is higher by a predetermined value than evaporation pressure that corresponds to the temperature at which the outdoor heat exchanger is frosted (0.70 MPa at 0° C.) is set as threshold pressure. The rotational speed of the outdoor fan is controlled such that the evaporation pressure during the heating operation does not become equal to or less than this threshold pressure.
In the above control, the evaporation pressure is periodically detected (detected by using a pressure sensor that is provided on a suction side of the compressor). If the detected evaporation pressure is equal to or less than the threshold pressure, the rotational speed of the outdoor fan is increased. Accordingly, an amount of ambient air flowing into the outdoor heat exchanger is increased, and evaporation capacity is increased. Thus, the evaporation pressure is increased. When the evaporation pressure becomes higher than the threshold pressure, the current rotational speed of the outdoor fan is maintained and controlled such that the evaporation pressure does not become equal to or less than the threshold pressure.